In recent years, accompanying the progress of the electronic device industry, demands for flexible printed wiring boards for mounting electronic parts such as ICs and LSIs have rapidly increased. Thus, the down sizing, that is, the size and weight reduction, and functional enhancement of electronic devices themselves have markedly progressed. For this purpose, packaging systems using flexible film carrier tapes, such as TAB (tape automated bonding) tapes, T-BGA (ball grid array) tapes, and ASIC (application specific integrated circuit) tapes, have been adopted. Particularly, in electronic devices having Liquid crystal displays, such as personal computers and mobile telephones, the progress toward high precision and thickness reduction is sharp, and flexible printed wiring boards having fine-pitch circuits formed thereon, which has not been realized conventionally, have been demanded; furthermore, there is an urgent need for the establishment of a method for mounting ICs and the like thereon.
In order to satisfy the demand for downsizing of electronic devices as described above, the technique for enabling ICs and the like to be mounted in narrow spaces is required. The techniques recently attracting attention include a packaging system known as chip on film (hereafter abbreviated as COF). In COF, a laminate of a copper foil (conductive layer), which is a conductive material forming a conductive circuit, and a film consisting of an insulating material such as polyimide (insulating layer)) is used; and the laminated film (hereafter referred to as flexible laminated board) is subjected to etching treatment to form a conductive circuit to be a flexible printed wiring board, whereon IC chips are directly mounted to perform packaging (COF).
A flexible printed wiring board used in the COF has no device holes as in a conventional TAB tape, and no inner leads corresponding to the conductive circuit portion not supported by the film exist. Namely, since the conductive circuit (even if it is an inner lead) is always in the state supported by the film, the line space of the conductive circuit corresponding to the inner lead of the TAB tape can be made further finer. The reason is that since the film is supported, the strength of the conductive circuit required during bonding can be secured even if the conductive circuit is made finer.
Since the flexible printed wiring board for COF has no device holes, the alignment in mounting IC or the like is performed by radiating light onto the conductive circuit pattern from one direction, confirming the transmitted light of the film on the opposite side, and directly recognizing the conductive circuit pattern shape. Since the alignment method using transmitted light can be applied to conventionally employed mounting machines for TAB tapes, the use of expensive equipment such as a chip bonder exclusively used in COF is not required, and the method tends to be widely adopted. For these reasons, a flexible printed wiring board for COF is required to have optical transmittance of the degree to exactly recognize alignment patterns.
As a flexible printed wiring board used for COF, a two-layer type board is known as obtained using a method wherein a seed layer of a metal such as nickel is formed on the surface of the film of a polyimide resin or the like (insulating layer), and copper to be a conductor is plated on the metal seed layer. In the flexible printed wiring board obtained using this method, known as a direct metallization method, since the insulating layer constituted by a polyimide resin or the like is formed in a relatively transparent state and has a high light transmittance, alignment for IC mounting can easily be performed; however, it has been pointed out that the adhesion between the formed conductor circuit and the insulating layer (peel strength) is low, and tends to cause migration.
In addition, it has been known that other flexible printed wiring boards can be manufactured using a casting method wherein a polyimide resin or the like as an insulating layer is applied to the surface of a conductor such as copper foil; or the most popular lamination method wherein a copper foil is bonded on such a film as a polyimide resin as the conductive layer for forming the conductive circuit. The flexible printed wiring board obtained using these casting method or lamination method excels in adhesion between the insulating layer and the conductive layer, and has high migration resistance compared with the direct metallization method. However, since the insulating layer on the portion from which the copper foil, i.e., the conductive layer, was etched off became the replica of the adhering surface of the copper foil, it tended to scatter light, and there might be the case where alignment using transmitted light could not be performed in IC mounting.
Therefore, it is the current status that when a flexible printed wiring board produced by the casting method or lamination method is used for COF, alignment holes must be separately formed in the insulating layer such as a polyimide resin by laser processing or the like in addition to the etching treatment for forming the conductive circuit.
To solve such problems, the present applicant has proposed a laminated film and a film carrier tape suitable for COF applications (e.g., refer to Patent Reference 1, Japanese Patent Laid-Open No.2003-23046). This laminated film has a structure wherein a conductive layer and an insulating layer are laminated, and is characterized in that the optical transmittance of the insulating layer in the region from which the conductive layer has been etched off is 50% or more. In addition, the copper foil for forming the conductive layer has produced a laminated film having a high optical transmittance by setting the surface roughness of the adhering surface thereof at 0.1 to 1.8 μm. Such a laminated film and film carrier tape can perform a good alignment in mounting ICs and the like.
However, when a copper foil having a reduced surface roughness of the adhering surface is used, the adhesion between the conductive layer and the insulating layer, that is, the lowering of the peel strength of the conductive circuit, cannot be avoided, and the laminated film having a desired peel strength must be manufactured after various examinations, such as the type of the resin constituting the insulating layer, and the surface treatment method including surface roughness on the adhering surface of the copper foil. Although the optical transmittance of the laminated film can be controlled to some extent by controlling the surface roughness on the adhering surface of the copper foil, it is the present situation that there are no findings on parameters for quality control of the adhering surface of the electrodeposited copper foil in order to realize the laminated film having a predetermined optical transmittance and a good adhesion, and the early clarification is required.
The present invention is devised in the above-described situations, and the object of the present invention is to provide a flexible printed wiring board suitable for COF that realizes a high optical transmittance of the insulating layer on the constitution of the flexible printed wiring board, and excels in the adhesion between the conductive layer and the insulating layer, and migration resistance.